Alkalis as Sources of Hydroxide Ions

Introduction

If acids are defined by the hydrogen ions they release, what defines their chemical opposites? Sodium hydroxide solution, limewater and household ammonia all feel soapy, turn red litmus blue and cancel out acids. The particle they have in common is the hydroxide ion , OH⁻. On this page you will also sort out two words that are often muddled: base and alkali .

Core explanation

Bases. A base is any substance that reacts with an acid to form a salt and water. Bases include metal oxides (such as copper(II) oxide, CuO), metal hydroxides (such as sodium hydroxide, NaOH) and, in a broader sense, metal carbonates and ammonia.

Alkalis. An alkali is a base that dissolves in water and produces hydroxide ions, OH⁻(aq). So every alkali is a base, but not every base is an alkali. Copper(II) oxide neutralises sulfuric acid, but it is insoluble in water, so it is a base and not an alkali.

How hydroxide ions form. Soluble metal hydroxides are ionic solids. When they dissolve, the ions separate:

NaOH(s) → Na⁺(aq) + OH⁻(aq)

KOH(s) → K⁺(aq) + OH⁻(aq)

Calcium hydroxide is only slightly soluble, but the part that dissolves releases two hydroxide ions per formula unit:

Ca(OH)₂(s) → Ca²⁺(aq) + 2OH⁻(aq)

Some metal oxides react with water to make hydroxides. Sodium oxide, for example:

Na₂O(s) + H₂O(l) → 2NaOH(aq)

Ammonia gas is a special case. It contains no hydroxide, but it takes an H⁺ from a water molecule, leaving OH⁻ behind:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Common alkalis. Sodium hydroxide, potassium hydroxide, calcium hydroxide (limewater) and aqueous ammonia are the alkalis you will use most. Group 1 hydroxides are very soluble; most transition-metal hydroxides and oxides are insoluble, so they are bases but not alkalis.

Alkaline properties. Because they contain OH⁻(aq), alkaline solutions have pH above 7, turn red litmus blue and universal indicator blue or purple, and neutralise acids. Concentrated alkalis are corrosive: they break down fats and proteins, which is why they feel slippery on skin — they are actually attacking it.

Step-by-step reasoning

To decide whether a substance is an alkali:

1. Ask whether it reacts with an acid to give a salt and water. If yes, it is a base. 2. Ask whether it dissolves in water. If not, it is a base but not an alkali. 3. If it dissolves, check that the solution contains OH⁻(aq) (pH above 7). If so, it is an alkali.

Visual explanation

Draw a large circle labelled "bases". Inside it, draw a smaller circle labelled "alkalis". Put CuO, MgO and Fe(OH)₂ in the outer ring, and NaOH, KOH, Ca(OH)₂ and NH₃(aq) inside the small circle. The diagram shows at a glance that alkalis are a subset of bases.

Real-world analogy

Bases and alkalis are like fruit and citrus fruit. Every orange is a fruit, but not every fruit is an orange. In the same way, every alkali is a base, but many bases, such as copper(II) oxide, are not alkalis.

Real-world example

Oven cleaners often contain sodium hydroxide. Hydroxide ions react with baked-on grease, converting fats into soap-like substances that can be wiped away. For this reason oven cleaners carry corrosive hazard symbols, and gloves and eye protection are recommended when using them.

Why?

Why do all alkalis share properties even though their formulae differ? Once dissolved, they all supply the same particle, OH⁻(aq). The metal ions that accompany the hydroxide, such as Na⁺ or K⁺, are spectators. Neutralising an acid, turning litmus blue and raising pH are all reactions of the hydroxide ion itself.

Common misconception

"All bases dissolve in water." Many important bases are insoluble, including copper(II) oxide, iron(III) hydroxide and magnesium oxide. They still neutralise acids, but because they do not dissolve they cannot change the colour of indicator paper dipped into water.

Worked example

Question: Barium hydroxide, Ba(OH)₂, dissolves in water. Write the equation for its dissolving and state how many moles of OH⁻ ions are produced by 0.5 mol of Ba(OH)₂.

Reasoning: Each formula unit contains one Ba²⁺ and two OH⁻. The ions separate when it dissolves, so moles of OH⁻ = 2 × moles of Ba(OH)₂.

Answer: Ba(OH)₂(s) → Ba²⁺(aq) + 2OH⁻(aq); 2 × 0.5 = 1.0 mol OH⁻.

Quick check

1. Is copper(II) oxide a base, an alkali or both? Explain briefly. Answer: It is a base only, because it neutralises acids but does not dissolve in water.

Exam focus

A precise definition earns the mark: "an alkali is a base that dissolves in water to form hydroxide ions". Examiners often ask you to classify substances; check solubility before calling anything an alkali. Include (aq) on OH⁻.

Advanced insight

In the Brønsted–Lowry model a base is a proton acceptor. Hydroxide ions accept protons to form water, and ammonia accepts protons to form ammonium ions. Even oxide ions, O²⁻, are powerful proton acceptors: O²⁻ + H₂O → 2OH⁻. This explains why soluble metal oxides make alkaline solutions without containing hydroxide to begin with.

Summary

A base reacts with an acid to form a salt and water. An alkali is a base that dissolves in water to produce hydroxide ions, OH⁻(aq). Soluble hydroxides release OH⁻ as their ions separate, some oxides react with water to form hydroxides, and ammonia produces OH⁻ by taking H⁺ from water. All alkalis are bases, but insoluble bases such as CuO are not alkalis.

Practice questions

1. Define an alkali. Answer: A base that dissolves in water to produce hydroxide ions, OH⁻(aq). 2. Write the equation for potassium hydroxide dissolving in water. Answer: KOH(s) → K⁺(aq) + OH⁻(aq). 3. Magnesium oxide neutralises hydrochloric acid but is almost insoluble in water. Classify it and explain. Answer: It is a base but not an alkali, because it neutralises an acid but does not dissolve to give OH⁻ ions. 4. Explain how ammonia solution can contain hydroxide ions even though NH₃ has no oxygen. Answer: Ammonia takes an H⁺ ion from a water molecule, forming NH₄⁺ and leaving OH⁻ behind.